My screensaver stops working. How do I fix that?

My screensaver and the sleep mode suddenly stop working after 10-20 seconds, and my Mac comes back to life, both when leaving the computer still and when using a hot corner.

I have tried deleting the com.apple.screensaver.plist as well as the preferences of the single screensavers, I have tried unplugging my devices, I have tried checking permissions and closing all applications, as well as checking the Activity Manager. The only way to get the screen to sleep is to close the laptop.

How can I get it back working?

I am using Snow Leopard 10.6.8 on a MacBook Pro with 2.3 GHz Intel Core i5